Internal details of std::sort() in C++

Sorting is one of the most basic functions applied to data. It means arranging the data in a particular fashion, which can be increasing or decreasing. There is a built-in function in C++ STL by the name of sort().

std::sort() is a generic function in C++ Standard Library, for doing comparison sorting.

Syntax:

sort(startaddress, endaddress, comparator)

where:
startaddress: the address of the first element of the array
endaddress: the address of the last element of the array
comparator: the comparison to be done with the array. 
            This argument is optional.

Example:

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// C++ program to demonstrate 
// behaviour of sort() in STL. 
  
#include <bits/stdc++.h> 
using namespace std; 
    
int main() 
    int arr[] = {1, 5, 8, 9, 6, 7, 3, 4, 2, 0}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
    
    sort(arr, arr+n); 
    
    cout << "\nArray after sorting using "
         "default sort is : \n"
  
    for (int i = 0; i < n; ++i) 
        cout << arr[i] << " "
    
    return 0; 

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Output:

Array after sorting using default sort is : 
0 1 2 3 4 5 6 7 8 9

Time Complexity

  • Best Case – O(N log N)
  • Average Case- O(N log N)
  • Worse Case- O(N log N)

where, N = number of elements to be sorted.

Algorithms used by sort()

The algorithm used by sort() is IntroSort. Introsort being a hybrid sorting algorithm uses three sorting algorithm to minimise the running time, Quicksort, Heapsort and Insertion Sort. Simply putting, it is the best sorting algorithm around. It is a hybrid sorting algorithm, which means that it uses more than one sorting algorithms as a routine.



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/* A Program to sort the array using Introsort.
  The most popular C++ STL Algorithm- sort()
  uses Introsort. */
  
#include<bits/stdc++.h>
using namespace std;
  
// A utility function to swap the values pointed by
// the two pointers
void swapValue(int *a, int *b)
{
    int *temp = a;
    a = b;
    b = temp;
    return;
}
  
/* Function to sort an array using insertion sort*/
void InsertionSort(int arr[], int *begin, int *end)
{
    // Get the left and the right index of the subarray
    // to be sorted
    int left = begin - arr;
    int right = end - arr;
  
    for (int i = left+1; i <= right; i++)
    {
        int key = arr[i];
        int j = i-1;
  
       /* Move elements of arr[0..i-1], that are
          greater than key, to one position ahead
          of their current position */
        while (j >= left && arr[j] > key)
        {
            arr[j+1] = arr[j];
            j = j-1;
        }
        arr[j+1] = key;
   }
  
   return;
}
  
// A function to parition the array and return
// the partition point
int* Partition(int arr[], int low, int high)
{
    int pivot = arr[high];    // pivot
    int i = (low - 1);  // Index of smaller element
  
    for (int j = low; j <= high- 1; j++)
    {
        // If current element is smaller than or
        // equal to pivot
        if (arr[j] <= pivot)
        {
            // increment index of smaller element
            i++;
  
            swap(arr[i], arr[j]);
        }
    }
    swap(arr[i + 1], arr[high]);
    return (arr + i + 1);
}
  
  
// A function that find the middle of the
// values pointed by the pointers a, b, c
// and return that pointer
int *MedianOfThree(int * a, int * b, int * c)
{
    if (*a < *b && *b < *c)
        return (b);
  
    if (*a < *c && *c <= *b)
        return (c);
  
    if (*b <= *a && *a < *c)
        return (a);
  
    if (*b < *c && *c <= *a)
        return (c);
  
    if (*c <= *a && *a < *b)
        return (a);
  
    if (*c <= *b && *b <= *c)
        return (b);
}
  
// A Utility function to perform intro sort
void IntrosortUtil(int arr[], int * begin,
                  int * end, int depthLimit)
{
    // Count the number of elements
    int size = end - begin;
  
      // If partition size is low then do insertion sort
    if (size < 16)
    {
        InsertionSort(arr, begin, end);
        return;
    }
  
    // If the depth is zero use heapsort
    if (depthLimit == 0)
    {
        make_heap(begin, end+1);
        sort_heap(begin, end+1);
        return;
    }
  
    // Else use a median-of-three concept to
    // find a good pivot
    int * pivot = MedianOfThree(begin, begin+size/2, end);
  
    // Swap the values pointed by the two pointers
    swapValue(pivot, end);
  
   // Perform Quick Sort
    int * partitionPoint = Partition(arr, begin-arr, end-arr);
    IntrosortUtil(arr, begin, partitionPoint-1, depthLimit - 1);
    IntrosortUtil(arr, partitionPoint + 1, end, depthLimit - 1);
  
    return;
}
  
/* Implementation of introsort*/
void Introsort(int arr[], int *begin, int *end)
{
    int depthLimit = 2 * log(end-begin);
  
    // Perform a recursive Introsort
    IntrosortUtil(arr, begin, end, depthLimit);
  
      return;
}
  
// A utility function ot print an array of size n
void printArray(int arr[], int n)
{
   for (int i=0; i < n; i++)
       printf("%d ", arr[i]);
   printf("\n");
}
  
// Driver program to test Introsort
int main()
{
    int arr[] = {3, 1, 23, -9, 233, 23, -313, 32, -9};
    int n = sizeof(arr) / sizeof(arr[0]);
  
    // Pass the array, the pointer to the first element and
    // the pointer to the last element
    Introsort(arr, arr, arr+n-1);
    printArray(arr, n);
  
    return(0);
}

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Output:

-313 -9 -9 1 3 23 23 32 233

Standard C library provides qsort() that can be used for sorting an array. As the name suggests, the function uses QuickSort algorithm to sort the given array
It is better to use sort() instead of qsort() because:

  1. sort() does not use unsafe void pointers like qsort().
  2. qsort() makes large number of function calls for comparison function compared to sort().
  3. C++ code with sort() is relatively faster than code with qsort().

Detailed article : Comparison of sort() with qsort()



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